These URL schemes are defined in RFC 1738, and are used for logging in via a terminal emulator. They have the form
telnet://user:password@host:port
but the password component is ignored. By default, the
telnet scheme is handled via Tramp (see URL Types Supported via Tramp).
To handle telnet and tn3270 URLs, a telnet or tn3270
(the program names and arguments are hardcoded) session is run in a
terminal-emulator buffer. Well-known ports are used if the URL
does not specify a port.